1. FIELD OF THE INVENTION
The present invention relates to a frequency control apparatus of a multi-refrigeration cycle system. More particularly, it relates to such apparatus for controlling the frequency of an electric current to be supplied to a motor to control driving speed.
2. DISCUSSION OF THE BACKGROUND
FIG. 3 is a block diagram showing a conventional speed control apparatus for a motor used for an air conditioning apparatus which is disclosed, for instance, in Japanese Unexamined Patent Publication No. 28902/1986.
In FIG. 3, a reference numeral 1 designates an input control device which specifies an operating mode for an air conditioning apparatus, a numeral 2 designates a target temperature setting device for setting a temperature in a room, a numeral 3 designates a room temperature sensor, a numeral 4 designates a logical operation unit which comprises a temperature error detector 5, an initial speed generator 6, a drive/stop signal generator 7, a temperature-error-change detector 8, a temperature-error-upper-limit detector 9, a driving speed determining device 10 and a timer 11. A numeral 12 designates a motor driving controller, and a numeral 13 designates a motor.
The operation of the conventional speed control apparatus will be described. The logical operation unit 4 receives a signal of the operating mode such as cooling or warming operation and a drive/stop instruction signal to the air conditioning apparatus which are provided by the input control device 1, a signal on the information of target room temperature which is provided from the temperature setting device 2 and a digital signal converted from an analogue output signal from the room temperature sensor 3, and conducts logic operations on the basis of the input signals. As a result of the logic operations, the logical operation unit 4 outputs a drive/stop signal and a driving speed signal to the motor driving controller 12 so that the operation of the motor 13 is controlled.
In the conventional speed control apparatus, since the output signal of the target room temperature by the target temperature setting device 2 and the output signal by the room temperature sensor 3 were respectively inputted in the logical operation unit 4, there was provided no signal with respect to a primary current to be supplied to the motor 13. Accordingly, operations of the speed control apparatus were often stopped when an over current preventing means was actuated. For this purpose, it was necessary to determine an upper limit of frequency on the basis of a temperature level. Further, it was necessary to prevent an excessive primary current from flowing when an excessive load caused by, for instance, a liquid-back phenomenon was applied to the motor. Also, the conventional speed control apparatus has a disadvantage that a load was rapidly increased by reduction of the revolution speed of the fan in a room side heat exchanger so that the primary current exceeded its upper limit. Further, when a plurality of room side units were operated, it was impossible to treat the signals of the target room temperature setting devices and the room temperature sensors.
Further, in the conventional speed control apparatus, when the frequency of a current for driving the motor was increased in response to an error signal between a room temperature and a target temperature, a pressure produced in the multi-referigeration cycle system exceeded a predetermined upper limit pressure to thereby cause stop of the compressor by actuation of a device which prevents production of a high pressure. For this purpose, it was necessary to predetermine the upper limit of the maximum frequency depending on a temperature level. In addition, it was necessary to prevent a pressure from increasing when an excessive load caused by, for instance, the liquid-back phenomenon was applied to the compressor. Further, the conventional speed control apparatus had disadvantages that when revolution speed of the fan in the room side heat exchanger was decreased, a load was rapidly increased, so that a pressure produced exceeded a predetermined upper limit value, and when a plurality of room side heat exchangers were operated, it was impossible to treat the signals of the target temperature setting devices and the room temperature sensors.